According to the earlier Finnish patent application No. 850208, a similar monitoring of the cooking process is based on an indirect procedure, by following the development of the proportion of aliphatic acids originating from the carbohydrate material in the wood and dissolved in the cooking liquor, said proportion in its turn bearing a relation to the dissolving of lignin. The procedure of the present invention is based on those regularities which have now been detected in the degradation of lignin, and on the means by which these degradation products can be analysed during the cooking process.
It is known in the art that when in alkaline cooking processes the lignin present in the wood is removed, part of the carbohydrate material in wood is also degraded (Sjostrom, E., Wood Chemistry, Fundamentals and Applications, Academic Press, New York, 1981). The organic matter dissolved in the spent liquor is thus composed not only of lignin fragments but also of carbohydrate degredation products and extractives from the wood. The lignin fraction of the spent liquor, in turn, is rather heterogeneous, consisting of various-sized molecules, part of which are in the monomer stage (Gierer, J. & Lindberg, O., Acta Chem. Scand. B 34(3) (1980)161, and Niemela, K. & Sjostrom, E., Holzforschung 40(1986)361). Said monomers are mainly phenol derivatives having a structure with a varying number and position of the groups substituted in the benzene ring. The formation of said monomeric phenols is essentially influenced by the cooking conditions applied in each instance, and by the raw material which is used. Thus, for instance, when birch wood is being cooked, also partly different lignin decomposition products are formed from those resulting when softwood is similarly cooked.